Pulverulent intermetallic materials for the reversible storage of hydrogen

ABSTRACT

The present invention relates to pulverulent materials suitable for storing hydrogen, and more particularly to a method of preparing such a material, in which: (A) a composite metallic material having a specific granular structure is prepared by co-melting the following mixtures: a first metallic mixture (m1), which is an alloy (a1) of body-centred cubic crystal structure, based on titanium, vanadium, chromium and/or manganese, or a mixture of these metals in the proportions of the alloy (a1); and a second mixture (m2), which is an alloy (a2), comprising 38 to 42% zirconium, niobium, molybdenum, hafnium, tantalum and/or tungsten and 56 to 60 mol % of nickel and/or copper, or else a mixture of these metals in the proportions of the alloy (a2), with a mass ratio (m2)/(m1+m2) ranging from 0.1 wt % to 20 wt %; and (B) the composite metallic material thus obtained is hydrogenated, whereby the composite material is fragmented (hydrogen decrepitation).

The present invention relates to the storage of hydrogen on anindustrial scale. It relates more specifically to materials which permita reversible storage of s hydrogen in the form of metal hydrides.

Hydrogen (H₂) is used in numerous industrial fields, especially as afuel (for example, in heat engines or fuel cells), or also as a reagent(typically for hydrogenation reactions). In this context, bearing inmind its volume in the gaseous state and its explosiveness, it isdesirable for hydrogen to be stored in a form ensuring a small spacerequirement and safe containment.

Hydrogen is usually stored under pressure (so-called hyperbaric storageat pressures typically of the order of from 20 to 70 MPa) or in liquidform (at temperatures lower than or equal to 20.4 K). These storagemethods are generally found to be expensive, particularly in terms ofenergy. In addition, they do not enable safety requirements to be fullysatisfied, especially in the case of storage under pressure.

More advantageously, it has been proposed to store hydrogen in the formof hydrides. In this context, the hydrogen to be stored is typicallybrought into contact with a metallic material (generally an alloy) underpressure and temperature conditions which cause the hydrogen to beincorporated in atomic form in the crystal lattice, by conversion of themolecular hydrogen H₂ into a hydride (so-called hydrogen “charging”step). In order to recover the hydrogen thus stored, conditions of lowerpressure and/or higher temperature, which promote the reverse reaction(hydrogen “discharge”), are required. In this context, it is possible todetermine a “reversible storage capacity”, expressed as a percentage bymass, which corresponds to the maximum amount of hydrogen which can bedischarged by the storage material once it has been charged. For moredetails on the storage of hydrogen in the form of hydrides, referencemay be made especially to Hydrogen In Intermetallic Compounds I and II,L. Schlapbach, Springer-Verlag, (1988).

Compared with the above-mentioned storages under pressure or at very lowtemperature, the above-mentioned storage of hydrogen in the form ofhydrides permits safer storage, with a smaller space requirement andgenerally a lower cost, especially in terms of energy. Furthermore,hydrogen freed from hydrides has the advantage of being in aparticularly pure form, which makes it especially suitable for use indevices of the fuel cell type or in fine chemistry reactions where it isdesired to be free from the presence of impurities to the maximumextent.

In particular, the use of materials having a crystal structure of typeAB₂ (such as ZrCr₂), or also materials comprising alloys of the typeFeTi or LaNi₅, as Da metal compounds capable of ensuring the storage ofhydrogen in the form of hydrides has been described (in this connection,reference may be made especially to the above-mentioned work Hydrogen inIntermetallic Compounds I and II). However, those materials are oflimited interest because, although they lead to the above-mentionedadvantages, they have mediocre performances, is especially in terms ofreversible storage capacity. In particular, although the alloy FeTi isrelatively inexpensive, it has a low reversible storage capacity (of theorder of 1% by mass), which means that it is used only very selectively,for heavy-duty applications (for example in submarines). The LaNi₅alloys for their part have much higher manufacturing costs with areversible storage capacity which is still low (generally of the orderof 1.4% by mass at the very most). As for materials having a crystalstructure of type AB₂, their reversible storage performance is generallylower than 1.8% by mass and, in addition, they usually have stabilityproblems after a few cycles of hydrogen charging and discharging.

More advantageously, in order to effect the reversible storage ofhydrogen in the form of hydrides, it has been proposed to use alloyshaving a body-centred cubic crystal structure (referred to hereinafteras “B.C.C. alloys”), for example, alloys having the general formulaTiVCr or TiVMn. Such B.C.C. alloys and their use in the storage ofhydrogen have been described, in particular, by S. W. Cho, C. S. Han, C.N. Park, E. Akiba, in J. Alloys Comp., vol. 294, p. 288, (1999), or byT. Tamura, M. Hatakeyama, T. Ebinuma, A. Kamegawa, H. Takamura, M.Okada, in J. Alloys Comp., vol 505, pp. 356-357 (2003). B.C.C. alloystypically enable hydrogen to be stored with a reversible storagecapacity which may reach values of the order of 2.5% by mass, or evenmore. Furthermore, these B.C.C. alloys are generally still efficientafter several charging and discharging cycles. Moreover, the conditionsof use of B.C.C. alloys are particularly advantageous inasmuch as thecharging and discharging of these materials can be effected attemperatures from ambient temperature (typically 15 a 25° C.) to 100°C., without having to use hydrogen pressures higher than 1 MPa, whichmakes them the materials of choice for hydrogen storage.

Nevertheless, despite these various advantages, B.C.C. alloys generallyhave a relatively low reactivity, with reduced kinetics of hydrogencharging and discharging. Thus, it is typically observed that a solidingot of a B.C.C. alloy having a volume of 2 cm³ absorbs a maximum ofthe order of 0.1% by mass of hydrogen even when it is placed under ahigh hydrogen pressure and at high temperatures, for example at 3 MPaand at 250° C. for 3 days. In order to achieve the above-mentionedreversible storage capacities of the order of 2.5% by mass or more, itis necessary to use the B.C.C. alloy in a form having an adequatespecific surface area capable of permitting a satisfactory exchangebetween the alloy and the hydrogen to be stored.

To that end, a preliminary mechanical milling of the alloys having abody-centred cubic crystal structure is generally carried out beforethey are brought into contact with the hydrogen. Apart from the factthat it is found to be expensive, both in terms of time and in terms ofenergy, such mechanical milling proves to be very difficult toimplement, bearing in mind the particularly high mechanical resistancewhich alloys having a body-centred cubic crystal structure generallyhave. In fact, such mechanical milling only permits the production ofcoarse powders, unless particularly elaborate and expensive conditionsof the type of the ball milling or melt spinning techniques are used,these being incompatible with a quantitative alloy preparation which canreasonably be exploited on an industrial scale. Thus, the powdersobtained in accordance with the milling techniques applicable on anindustrial scale typically have, at best, a grain size of the order offrom 300 μm to 500 μm.

This grain size is found to be unsatisfactory in bringing about a reallyefficient incorporation of hydrogen, and a subsequent activation of thematerial is usually required, in particular in order further to improvethe specific surface area. This activation generally uses a treatment ofthe alloy at high temperatures. Typically, it is recommended to subjectthe powder to an activation pretreatment, for example, in accordancewith a method of the same type as those described by Cho et al. in J.Alloys Comp. vol. 45, pp. 365-357, (2002), which consists in subjectingthe powder to the following conditions:

-   -   treatment under high vacuum at 500° C. for 1 hour;    -   cooling to ambient temperature;    -   placing under a hydrogen pressure of 5 MPa for 1 hour;    -   repetition of the above steps at least three times; and    -   placing under a final vacuum at 500° C. for one hour.        The necessity for such pretreatment of the powder makes the        storage process even more burdensome, which is reflected, in        particular, in terms of costs.

An object of the present invention is to provide a material for thestorage of hydrogen, which is at least as efficient, and preferably moreefficient, than the pretreated B.C.C. alloy powders of theabove-mentioned type, and which is also accessible in accordance with amethod of synthesis which is less expensive and simpler than theprocesses for the preparation of the B.C.C. alloy powders that arecurrently known.

In this context, the invention aims in particular to provide anindustrially exploitable process which permits the direct and simpleproduction of a material that is efficient in the storage of hydrogenwithout having to use the pretreatment steps necessary for theproduction of the reactive B.C.C. alloy powders that are currentlyknown.

To that end, according to a first aspect, the present invention providesa specific process permitting the preparation of a pulverulent materialsuitable for the storage of hydrogen. More precisely, this processcomprises the following steps:

-   (A) the preparation of a composite metallic material, by co-melting,    then cooling, the following metallic mixtures:    -   a first metallic mixture (m1), which is:        -   an alloy (a1), having a body-centred cubic crystal            structure, comprising titanium (Ti), vanadium (V), and            another metal M selected from chromium (Cr), manganese (Mn),            and mixtures of those metals; or        -   a mixture of the constituent metals of the alloy (a1), in            the proportions of that alloy, those metals being present in            the mixture in the state of isolated metals and/or in the            state of metal alloys; and    -   a second mixture (m2), which is:        -   an alloy (a2), comprising:            -   from 38 to 42 mole % of a first metal M¹ selected from                zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium                (Hf), tantalum (Ta), tungsten (W), and mixtures of those                metals; and            -   from 56 to 60 mole % of a second metal M², selected from                nickel (Ni), copper (Cu), and mixtures of those metals;                or        -   a mixture of the constituent metals of the alloy (a2), in            the proportions of that alloy, those metals being present in            the mixture in the state of isolated metals and/or in the            state of metal alloys;-   with a ratio by mass (m2)/(m1+m2) ranging from 0.1% to 20% by mass    in the co-melting step;-   and-   (B) a hydrogenation of the composite metallic material obtained,    permitting the conversion of at least a part of the metals present    into metal hydrides, and leading to a fragmentation of the material    in the form of a powder.

In the sense in which it is used here, the expression “metallic mixture”denotes, very generally, a composition or an association of compositionscomprising at least two metallic elements at their degree of oxidation0, these metallic elements being present in the state of isolated metalsand/or in the form of alloys. Such a metallic mixture does notnecessarily comprise the various metals and/or alloys in the intimatelymixed state. Thus, in accordance with one possible embodiment, themetals and/or alloys of a metallic mixture according to the inventionmay comprise a simple association of a first metal or alloy and a secondmetal or alloy, in the form of distinct batches, for example, in theform of physically distinct blocks. Nevertheless, it is usuallypreferred that the metals and/or metal alloys present in the mixtures(m1) and (m2) should be used in a divided form, for example, in thecrushed state, or even in the state of a finer powder, which enablesgood homogenization to be obtained more rapidly during the co-melting ofstep (A).

In the process of the invention, the mixtures (m1) and (m2) used may bealloys (a1) and (a2), or associations of metals and/or alloys having,overall, the same composition as those alloys.

In the context of the present description, “metal alloy” means amonophase or polyphase composition comprising several metallic elementsat their degree of oxidation 0, such as obtained after a joint meltingof these metals. Apart from these metallic elements, an alloy accordingto the invention may optionally contain other minority species (often indistinct minority phases in the alloy), such as non-metallic elements(C, S, O, N or B, for example) or also metallic elements in the oxidizedstate, these optional minority species then being present preferably ina proportion of less than 5% by mass, preferably in a proportion of lessthan 2% by mass, and more advantageously in a proportion of less than 1%by mass relative to the total mass of the alloy considered.

More precisely, the alloys (a1) and (a2) that can be used according tothe invention have the following specific characteristics.

The first alloy (a1) has a body-centred cubic crystal structure.Depending on the embodiment, this alloy may be monophase (alloy of thedefined intermetallic compound type) or polyphase. It is in any case analloy “comprising titanium, vanadium and another metal M (Cr and/orMn)”. This expression is understood to mean that the alloy (a1)contains, among other possible elements, titanium, vanadium and metal M(Cr and/or Mn), with oxidation state 0. These metallic elements (Ti, Vand the metal M) generally predominate (by mass and by mole) in thealloy (a1), where they generally represent at least 90%, usually atleast 95%, typically at least 98%, or even 99% by mass of the alloy(a1). In addition to these majority metallic constituents, the alloy(a1) may optionally contain other metallic elements, especially iron(Fe), cobalt (Co) or nickel (Ni).

According to one advantageous embodiment, the alloy (a1) corresponds tothe general formula (I) below:

Ti_(a)V_(b)M_(c)M′_(d)   (I)

wherein:

-   -   M has the above-mentioned meaning, and preferably denotes        chromium (Cr), or a mixture of chromium and manganese, with        manganese advantageously to predominating (by mole);    -   M′ denotes a metal or a mixture of metals, other than Ti, V or        Cr, for example selected from iron, cobalt, nickel, or mixtures        of those metals;    -   a is a number ranging from 0.05 to 2.5, typically from 0.1 to 2,        for example from 0.2 to 1.5;    -   b is a number ranging from 0.05 to 2.9, typically from 0.1 to        2.2;    -   c is a number ranging from 0.05 to 2.9, typically from 0.5 to        2.5; and    -   d, optionally zero, is a number ranging from 0 to 0.5, this        number preferably being less than 0.2, for example less than        0.1,        -   the sum (a+b+c+d) being equal to 3.

More specifically, it is found to be advantageous to use an alloy (a1)corresponding to the general formula (Ia) below:

Ti_(x)N_(y)Cr_(3-(x+y))   (Ia)

wherein:

-   -   x is a number ranging from 0.4 to 1, typically greater than or        equal to 0.5; and    -   y is a number ranging from 0.1 to 2.5, typically from 0.5 to        2.2, for example from 0.6 to 2,        -   the sum (x+y) being typically greater than 1.5, and            generally less than 2.5.

Alloys corresponding to the following general formulae:

TiV_(0.8)Cr_(1.2)

Ti_(0.9)V_(0.7)Cr_(1.4)

Ti_(0.833)V_(0.826)Cr_(1.334)

Ti_(0.7)V_(0.9)Cr_(1.4)

Ti_(0.66)VCr_(1.33)

Ti_(0.5)V_(1.9)Cr_(0.6)

may be mentioned, in a non-limiting manner, as examples of alloysparticularly suitable as the alloy (a1) according to the invention.

Regardless of its exact composition, an alloy (a1) of theabove-mentioned type can be prepared in accordance with methods knownper se. In general, such an alloy is prepared by melting a mixturecomprising, inter alia, titanium, vanadium and chromium and/or manganese(typically at temperatures of the order of from 1300 to 1700° C.). Thismelting is typically carried out by induction, usually under anatmosphere of a neutral gas (argon, for example), especially in order toprevent oxidation of the alloy.

The second alloy (a2) is for its part a monophase or polyphase metalalloy comprising the above-mentioned metals M¹ and M², which, in thiscase too, means that the alloy (a2) contains, inter alia, the metals M¹and M², with oxidation state 0, generally as majority elements (by massand by mole), these metals M¹ and M² usually being present in aproportion of a total amount representing at least 90%, generally atleast 95%, typically at least 98%, or even 99% of the total mass of thealloy (a2). The alloy (a2) may optionally contain other elements,especially metallic elements such as cobalt, preferably in a proportionof less than 5% by mass relative to the total mass of the alloy (a2).

Advantageously, the alloy (a2) comprises at least one phase comprising acompound having a crystal structure corresponding to the space groupAba2 or C2ca. In this context, the alloy (a2) is preferably asubstantially monophase alloy which is mainly constituted (namelytypically in a proportion of at least 95%, or even at least 98% and moreadvantageously at least 99% by volume) by such a crystalline compoundhaving an Aba2 or C2ca structure.

According to one particular embodiment, the alloy (a2) corresponds tothe formula (II) below:

M¹ _(7-m)M² _(10-n)M³ _(p)   (II)

wherein:

-   -   M¹ denotes a first metal, selected from zirconium (Zr), niobium        (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), tungsten        (W), and is mixtures of those metals, M¹ preferably being Zr;    -   M² denotes a second metal, selected from nickel (Ni), copper        (Cu), and mixtures of those metals, M² preferably denoting Ni;    -   M³ denotes a metal or a mixture of metals, optionally present in        the alloy, other than M¹ and M²;    -   m is a positive or negative number, or zero, ranging from −0.1        to +0.1;    -   n is a positive or negative number, or zero, ranging from −0.1        to +0.1;    -   p is a positive number, or zero, ranging from 0 to 0.2.

More preferably, the alloy (a2) corresponds to the formula (IIa) below:

M¹ ₇M² ₁₀   (IIa)

wherein M¹ and M² are as defined above.

In this context, the alloy (a2) advantageously corresponds to thefollowing formula:

Zr₇Ni₁₀

Regardless of its composition, the alloy (a2) can be prepared in asimilar manner to the alloy (a1), typically by joint melting of itsconstituent elements, generally at from 1100° C. to 1500° C., usually byinduction, advantageously under an atmosphere of a neutral gas, such asargon, in particular in order to prevent oxidation of the alloy.

In the process of the invention, the co-melting of the above-mentionedalloys (a1) and (a2), or, more generally, of metallic mixtures (m1) and(m2) comprising the metallic elements of those alloys in the proportionsof the alloys (a1) and (a2), is carried out with a ratio by mass(m2)/(m1+m2) of from 0.1 to 20%.

The inventors have now demonstrated that this co-melting of the metallicmixtures (m1) and (m2), in the above-mentioned ratio by mass, leads,after cooling, to a very specific composite metallic material which hasa novel property, namely that of fragmenting when subjected tohydrogenation (so-called hydrogen “crackling” phenomenon). Thisfragmentation leads to the production of a pulverulent material suitablefor hydrogen storage.

Surprisingly, it is found that the fragmentation of the material in theform of a powder occurs by itself during step (B), that is to say,simply by bringing about a hydrogenation of the material, and withoutrequiring any additional step. Thus, the implementation of steps (A) and(B) enables a material suitable for the storage of hydrogen to beobtained directly without having to use the expensive milling andpretreatment steps currently necessary for the preparation ofhigh-performance storage materials.

The process of the invention is therefore found to be particularly wellsuited to implementation on an industrial scale, and all the more so asits steps are carried out using conventional techniques of melting andhydrogenation of the same type as those conventionally used in the fieldof the industrial manufacture of hydrogen storage materials.

The composite metallic material obtained at the end of step (A) of theprocess of the invention has a very specific bi- or multiphase structurewhich is generally fine and homogeneous and which comprises:

-   -   a majority phase comprising titanium, vanadium and chromium        and/or manganese, dispersed in the form of grains typically        having dimensions of from 10 to 100 microns, especially from 20        to 80 microns, for example from 40 to 50 microns, this majority        phase having a body-centred cubic crystal structure; and    -   at least one intergranular phase comprising a first metal        selected from zirconium, niobium, molybdenum, hafnium, tantalum,        tungsten, or a mixture of those metals; and a second metal        selected from nickel, copper, or a mixture of those metals.

In this specific composite material, the majority-phase grains aredispersed in an intergranular medium comprising one or more phases.Thus, the intergranular phase(s) form walls between the dispersedgrains. These walls generally have an average thickness of the order ofa few microns (typically from 1 to 5 microns).

The above-mentioned composite material, obtained as an intermediate inthe process of the invention, is a material which has a novel structureand which, to the inventors' knowledge, has not been described hithertoand which, according to one particular aspect, constitutes a furthersubject of the present invention.

In this composite material, the majority phase (or “intragranular”phase) generally has a composition relatively similar to that of theinitial alloy (a1). The intergranular phase(s) comprise for their partusually the constituent metals of the alloy (a2). Nevertheless, itshould be noted that the co-melting of step (A) may bring aboutphenomena of diffusion of some atoms between the alloys, as a result ofwhich the compositions of the intragranular phase and the intergranularphase(s) may depart to a fairly great extent from the initialcompositions of the alloys (a1) and (a2). Likewise, the crystalstructure of the intragranular phase and the intergranular phase(s) maydiffer from those of the starting alloys. However, the intragranularmajority phase of the composite material systematically preserves thebody-centred cubic crystal structure of the starting alloy (a1).

It seems that the phenomenon of the crackling of the composite materialwhich was observed by the inventors, and which is turned to good accountin step (B) of the process, can be explained by the fact that theintergranular phases of the composite materials obtained in accordancewith step (A) are mechanically less resistant and more reactive tohydrogenation than is the intragranular majority phase having abody-centred cubic structure. It therefore seems that the hydrogenationof the material is more effective at the level of the intergranularphases than at the level of the intragranular phase, which brings aboutvery great mechanical stresses in the composite material, consequentlyembrittling the intergranular phases and ultimately leading to thefragmentation observed. In other words, in simple terms, thehydrogenation of step (B) may be regarded as leading to a separation ofthe grains of hydrogenated majority phase, the hydrogenation of thematerial bringing about an embrittlement of the intergranular phase suchthat the latter is no longer capable of acting as a grain binder.

The pulverulent material obtained after the hydrogen crackling of step(B) therefore generally comprises grains of hydrogenated majority phasewhich have essentially become separated from each other and which arecovered, completely or in part, by hydrogenated intergranular phase. Inthe most common case, the grain size of the powder obtained at the endof step (B) therefore corresponds substantially to the average size ofthe grains of the majority phase of the composite material resultingfrom step (A), that is to say, a grain size of the order is ofapproximately 10 microns, typically of the order of from 40 to 50microns. Thus, entirely unexpectedly, the inventors have nowdemonstrated that it is possible to obtain hydrogen storage materials inthe form of powders having a grain size much finer than those resultingfrom the usual milling operations carried out on B.C.C. alloys, in anextremely simple manner and, surprisingly, without any milling stepproving necessary.

The grain size of the powders that is obtained at the end of step (B) ofthe process of the invention is found to be particularly advantageousbecause, apart from the fact that it brings about a large specificsurface area optimizing the exchanges between the hydrogen and thestorage material, this particular grain size is found to be bothsufficiently small to permit good diffusion of the hydrogen in thematerial (larger grain sizes would involve bulkier particles where thisdiffusion would be slowed down) and nevertheless sufficiently large toinhibit corrosion and the pyrophoric phenomena to which particles ofsmaller size would be sensitive. It is entirely surprising that theprocess of the invention enables such optimization of the particle sizeto be obtained directly.

In addition, the work of the inventors has demonstrated that thecomposite materials as obtained at the end of step (A) of the process ofthe invention behave in a very particular manner during thehydrogenation of step (B), which confers on them improved propertiescompared with monophase alloys having a body-centred cubic structure,especially in terms of capacity by mass for the reversible storage ofhydrogen, reactivity and charging and discharging kinetics.

These improved properties can be explained at least in part by thepresence of the intergranular phases which optimize the absorptionproperties of the alloys of body-centred cubic structure which arepresent in the intragranular majority phase. In this context, the workof the inventors has, in particular, demonstrated that the intergranularphases of the composite materials as obtained at the end of step (A)exhibit a high reactivity to hydrogenation, and especially a very goodaptitude for decomposing molecular hydrogen H₂ into atomic hydrogen,coupled with excellent properties of diffusing the atomic hydrogen soformed, which promotes a very great diffusion of hydrogen around thecells of the majority phase during the hydrogenation of step (B).Therefore, the composite material reacts very rapidly to hydrogenationand it is observed, in particular, that the granular phase having abody-centred cubic structure is hydrogenated far more rapidly than ifuse were made only of this phase reduced to the state of a powder.

Thus, the intergranular phase of the composite materials of theinvention plays a dual role in the hydrogen-absorption properties of thematerials obtained according to the invention:

on the one hand, this intergranular phase is sufficiently brittle andreactive to bring about the crackling phenomenon of step (B), whichleads to the production of a pulverulent material having a specificsurface area and a grain size which are particularly well suited tosubsequent cycles of charging and discharging hydrogen; and

on the other hand, it constitutes an excellent vector for hydrogentowards the intragranular phase, which permits very efficienthydrogenation of the material as of the first hydrogenation.

Furthermore, the inventors have demonstrated that the pulverulantmaterial as obtained at the end of step (B) of the process of theinvention has a reversible storage capacity which generally remainsstable over time, and usually does so s even after a large number ofcharging and discharging cycles. In particular, the hydrogen storagematerials of the present invention are not subject to the demixingphenomena observed with some B.C.C. alloys, such as those described, forexample, by H. Itoh, H. Arashima, K Kubo, T. Kabutomori and K. Ohnishiin J. Alloys Comp, vol. 404-406, pp. 417-420, (2005), for which a rapiddegradation of the alloy structure by separation of the alloyed elementsis observed after a few charging and discharging cycles, which thenleads to a rapid loss of the reversible hydrogen storage properties.

By contrast, the hydrogen storage materials of the present invention isgenerally remain very stable over time, that is to say, without anymodification of their structure substantially affecting their reversiblestorage capacity. In fact, variations over the first 5 to 10 chargingand discharging cycles may be observed (but with losses which areusually small), the reversible storage capacity being substantiallystabilized in the subsequent cycles, without any appreciable loss ofcapacity, and this usually even after 100 charging and dischargingcycles, and even after 1000 cycles.

According to one particular embodiment of step (A) which is usuallyfound to be particularly suitable and advantageous, the alloys (a1) and(a2) themselves, rather than other metallic mixtures, are used in step(A) as metallic mixtures (m1) and (m2), respectively. This embodimentgenerally permits optimization of the various advantages mentioned aboveand, in particular, it leads to a particularly fine and homogeneousgrain size for the composite material resulting from step (A) and, defacto, for the powder obtained after the crackling of step (B). In thecontext of this variant, in order further to optimize the properties ofthe materials prepared, it is preferred to start from the mosthomogeneous alloys (a1) and (a2) possible. To that end, each of thealloys (a1) and (a2) is advantageously prepared by the induction-meltingof its constituents, preferably under an inert gas, typically byhigh-frequency induction in a device of the cold crucible type. In orderfurther to improve the homogeneity, the alloys may be subjected toseveral successive melting operations. Typically, three successivemelting steps generally lead to satisfactory homogenization.

According to a further possible embodiment for step (A), by contrast asingle metallic mixture (m1+m2) comprising the various constituentelements of the alloys (a1) and (a2) not alloyed in the form of thosealloys (a1) and (a2) is used as the metallic mixtures (m1) and (m2).This metallic mixture (m1+m2) is advantageouslly a mixture of puremetals which are generally introduced in the divided state, for example,in the form of crushed lumps or powders. This second variant of step (A)generally leads to a composite material which has a coarser structurethan that of the composite materials obtained according to the previousvariant, often with less optimized properties for hydrogen storage.Nevertheless, this second variant is economically more advantageous thanthe previous variant because it does not require the implementation ofthe two melting steps necessary for the preparation of the alloys (a1)and (a2), and it therefore enables both the cost in terms of energy andthe duration of the process to be reduced, which makes it an embodimentparticularly suitable for large-scale industrial use.

More generally, regardless of the variant used and the exact nature ofthe mixtures (m1) and (m2) used, it is advantageous to carry out steps(A) and (B) under the conditions set out hereinafter.

In step (A), the co-melting of the alloy (a1) and the alloy (a2) can becarried out in accordance with any known method, provided that thealloys are heated to beyond the melting temperature of the mixtureconsidered (generally known from the corresponding phase plot).Nevertheless, it is generally preferred to carry out this co-meltingoperation by induction heating, generally by high-frequency induction.This co-melting by induction is typically carried out in an inductioncrucible, advantageously in a device of the cold crucible type, which,in particular, enables the metallic phases to be stirred efficiently,thus facilitating the formation of the grain structure of the compositematerial. Typically, the co-melting of step (A) is carried out attemperatures of the order of from 1000 to 1800° C., for example from1100 to 1300° C. Furthermore, the co-melting of step (A) canadvantageously be carried out by using several successive meltingoperations. Generally, three successive melting steps in theabove-mentioned temperature ranges result in a co-melting which isparticularly well suited to the process of the invention.

An important feature in step (A) is the ratio by mass (m2)/(m1+m2). Thisratio, which reflects the respective proportions of the mixtures (m1)and (m2) used, determines in part the structure of the compositematerial obtained at the end of step (A). In particular so that theeffects of the presence of the intergranular phase should be assensitive as possible in the composite material obtained, this ratio(m2)/(m1+m2) is advantageously at least 0.5% by mass, preferably atleast 1% by mass, and more preferably at least 2% by mass. Nevertheless,in order to obtain particularly advantageous absorption properties, itis generally found to be preferable for the material to be composedsubstantially of majority phase. In this context, it is usuallypreferred for the ratio (m2)/(m1+m2) to be less than 15% by mass,advantageously less than 1Q% by mass, for example, less than 8%. Thus,according to one advantageous embodiment, the ratio (m2)/(m1+m2) in theco-melting step is from 3 to 6% by mass. Typically this ratio is of theorder of 4% by mass.

When the alloy (a2) is used as the mixture (m2) in step (A), it isoptionally possible to mill the alloy (a2) before the co-meltingoperation. Such milling, although not generally required, enables theamount of alloy (a2) introduced to be proportioned relative to the massof the mixture (m1), which proves to be advantageous when it is desiredto reach a predetermined ratio (m2)/(m1+m2). It should be noted that,bearing in mind the mechanical brittleness of the alloy (a2), it is mucheasier to mill this alloy (a2) than to mill an alloy having abody-centred cubic structure.

On the other hand, it is preferable not to use a step of milling thealloy (a2) when the latter is used as the mixture (m2). In this context,it should be emphasized that a step of milling the alloy (a2) is in noway required in order to obtain the composite material of step (A).

For its part, step (B) of the process of the invention can be carriedout in accordance with any method known per se, provided that it isimplemented under conditions of temperature and pressure sufficient tobring about the desired hydrogenation of the material. The temperatureand pressure conditions required for this purpose may vary to a fairlylarge extent depending on the exact nature of the composite materialwhich has been synthesized in step (A). However, step (B) does notusually have to be carried out at a temperature exceeding 150° C., norat a hydrogen pressure higher than 1 MPa. Thus, step (B) can usually becarried out under gentle hydrogenation conditions, especially at from 15to 100° C., and under a hydrogen pressure which is typically from 0.8 to1 MPa. This very easy hydrogenation of the composite material is yetanother advantage of the process of the present invention which isreflected in terms of reduced costs (low energy supply in particular)and increased safety (working under low pressure in particular), whichmakes the process particularly advantageous from an industrial point ofview.

According to another aspect, the present invention relates also to thepulverulent materials comprising metal hydrides, as obtained at the endof the process of the invention.

These materials generally have a grain size of from 20 to 100 microns,preferably from 30 to 60 microns, for example from 40 to 50 microns. The“grain size” referred to here corresponds to the average size of theparticles present in the pulverent material, as determined by lasergranulometry, typically by means of a granulometer of the usual type,such as the Malvern Mastersizer 2000.

Furthermore, the pulverulent materials as obtained according to theinvention typically have a specific surface area of from 0.01 to 0.1m²/g, usually of at least 0.2 m²/g.

As emphasized above, these pulverulent materials are particularlysuitable for the reversible storage of hydrogen in the form of hydrides.

They have a very high capacity for the reversible storage of hydrogen,which is usually greater than 2.5% by mass, or even higher than 3% bymass. They also have a high reactivity with respect to hydrogenation,with very high hydrogen charging and discharging kinetics, enablinghydrogen to be discharged from and recharged into the material inextremely short times of the order of a few minutes at the very most.Moreover, these materials have a very long shelf life, with very goodresistance to cycling.

Apart from these advantages, it should also be emphasized that the PCI(Pressure Composition Isotherm) graphs of the storage materials of theinvention also bring to light other advantageous characteristicscompared with those of the C.C. materials conventionally used atpresent. There are thus observed, in particular, a to plateau ofequilibrium of the material-hydrogen system which is flatter than in thecase of C.C. materials and at a slightly higher pressure, and also ahysteresis between the curve recorded during hydrogen absorption andthat recorded during the weaker hydrogen desorption, which, inparticular, enables variations in pressure and reduced temperatures tobe used in order to effect the charging and discharging of hydrogen.

The use of the pulverulent materials of the invention as material forthe reversible storage of hydrogen in hydride form constitutes yetanother subject of the present invention.

In this context, the materials of the invention act as reservoirs fromwhich hydrogen can be extracted on demand by placing the material underconditions of temperature and pressure sufficiently low to ensurehydrogen desorption.

This method of reversible storage is found to be particularlyadvantageous in fields requiring the supply of particularly purehydrogen flows under conditions of safety.

In particular, the materials of the invention can advantageously be usedas reservoirs for supplying gaseous hydrogen in the following fields:

-   -   Static fuel cells (domestic or industrial)    -   Fuel cells for integrated applications (portable)    -   On-board fuel cells (boats, submarines, heavy vehicles . . . )    -   Heat engines (reciprocating, coils, . . . )    -   Fine chemistry.

In the context of these uses, the hydrogen storage materials of theinvention are alternately “emptied” of their hydrogen (under conditionsof sufficiently low temperature and sufficiently high pressure) and then“filled” with hydrogen again (in hydride form, at a higher temperatureand/or at a lower pressure). The materials resulting from thedehydrogenation of the materials of the invention, which are in the formof a powder substantially free from hydrides and which are capable ofsubsequently being rehydrogenated, constitute, according to yet anotheraspect, a specific subject of the present invention.

The features and various advantages of the present invention will emergeeven more clearly in the light of the illustrative examples set outhereinafter.

EXAMPLES Example 1 Preparation of Pulverulent Materials for HydrogenStorage Step 1.1: Preparation of Alloys Comprising TiV and Cr (AlloysA1)

Various samples of alloy (referred to hereinafter as “alloys A1”),having a body-centred cubic structure, were synthesized by co-meltingtitanium, vanadium and chromium, which were introduced in variableproportions. The respective masses of the metals used in each case(m_(Ti), m_(V) and M_(Cr) respectively) are compiled in Table 1 below:

TABLE 1 Alloys A1 Formula m_(Ti) m_(V) M_(Cr) Alloy A1.1TiV_(0.8)Cr_(1.2) 6.338 g 5.397 g 8.624 g Alloy A1.2Ti_(0.9)V_(0.7)Cr_(1.4) 5.685 g 4.706 g 9.608 g Alloy A1.3Ti_(0.833)V_(0.826)Cr_(1.334) 5.269 g 5.562 g 9.169 g Alloy A1.4Ti_(0.7)V_(0.9)Cr_(1.4) 4.407 g 9.569 g 6.027 g Alloy A1.5Ti_(0.66)VCr_(1.33) 4.165 g 6.717 g 9.118 g Alloy A1.6Ti_(0.5)V_(1.9)Cr_(0.6) 3.150 g 12.743 g  4.107 g

Titanium (purity: 99.5%), vanadium (purity: 99.9%) and chromium (purity:99.9%) were used as starting materials in the form of lumps having anaverage size of the order of 0.5 cm³, which were obtained by crushinglarger lumps resulting from the metallurgical industry.

For the synthesis of each of the alloys A1, the mixture of metals wasco-melted in an induction crucible of the cold crucible type, having apower of 100 kW, at a temperature of from 1500 to 1700° C., under anargon atmosphere. In order to obtain a homogeneous composition, eachsample was subjected to 3 successive induction-melting operations eachlasting 3 minutes.

Step 1.2: Preparation of an Alloy Zr₇Ni₁₀ (Alloy A2)

An alloy A2 of formula Zr₇Ni₁₀ was prepared by co-melting 10.430 g ofzirconium (purity: 99.9%) and 9.750 g of nickel (purity: 99.99%), whichwere both in the form of lumps having an average size of the order of0.5 cm³ and were obtained by crushing larger lumps resulting from themetallurgical industry, in the above-mentioned cold crucible, at atemperature of from 1100 to 1300° C., under an argon atmosphere. In thiscase too, the sample was subjected to 3 successive induction-meltingoperations.

Step 1.3: Co-Melting of the Alloys

Each of the alloys A1 synthesized in step 1.1 was co-melted with thealloy A2 of step 1.2, with in each case, a ratio by mass A2/(A1+A2)equal to 4%. For this purpose, in each case 30 g of alloy of type (a1)and 1.2 g of alloy Zr₇Ni₁₀ were used.

The co-melting of the mixture was carried out in the above-mentionedcold crucible, at a temperature of from 1200 to 1300° C., under an argonatmosphere. In this case too, the sample was subjected to 3 successiveinduction-melting operations each lasting 3 minutes.

The composite metallic materials M1 to M6 indicated in Table 2 belowwere thus obtained:

TABLE 2 composite materials obtained by co-melting the alloys A1 and A2Composite Alloy A1 Alloy A2 Ratio by mass material used used A2/(A1 +A2) M1 TiV_(0.8)Cr_(1.2) Zr₇Ni₁₀ 4% M2 Ti_(0.9)V_(0.7)Cr_(1.4) Zr₇Ni₁₀4% M3 Ti_(0.833)V_(0.826)Cr_(1.334) Zr₇Ni₁₀ 4% M4Ti_(0.7)V_(0.9)Cr_(1.4) Zr₇Ni₁₀ 4% M5 Ti_(0.66)VCr_(1.33) Zr₇Ni₁₀ 4% M6Ti_(0.5)V_(1.9)Cr_(0.6) Zr₇Ni₁₀ 4%

Observation of the materials by electron microscopy confirmed the grainstructure in each case, with grains having a size of the order of from40 to 50 microns, and a thickness of the intergranular phase of theorder of a few microns. The crystal structure of the various phases wasdetermined by X-ray diffraction using a Siemens D-5000 diffractometerusing the radiation of the copper K_(α) line, which confirmed thepresence of an intragranular phase having a C.C. structure and thepresence of intergranular phases of the type Ti₂Ni and ZrCrNi.

Step 1.4: Treatment with Hydrogen

Each of the materials M1 to M6 obtained at the end of step 1.3 wassubjected to hydrogenation under a stream of hydrogen at a temperatureof from 20 to 40° C., at a pressure of 1 MPa, for 30 minutes, whichresulted in a fragmentation of the material in the form of a powder.

Starting from each of the materials M1 to M6, powders comprising metalhydrides P1 to P6, respectively, each having a very narrow grain-sizerange, centred on 40 microns, were thus obtained.

Example 2 Tests for the Reversible Storage of Hydrogen by the Materialsof Example 1

In order to demonstrate their reversible storage properties, each of thepowders P1 to P6 prepared in the previous Example was subjected to thefollowing conditions:

-   -   hydrogen desorption by heating at 150° C. for one hour under a        low vacuum (10⁻⁴ Pa)    -   subjection of the sample so obtained to three successive cycles        of absorption at 50° C., 1 MPa then desorption at 250° C., 2        kPa.    -   The PCI graphs obtained enabled the following reversible        hydrogen storage capacities to be determined (C_(H), as % by        mass):

Material P1 P2 P3 P4 P5 P6 C_(H) 2.1% 1.7% 1.9% 2.2% 2.2% 2.5%

1-17. (canceled)
 18. Process for the preparation of a pulverulentmaterial suitable for the storage of hydrogen, comprising the followingsteps: (A) the preparation of a composite metallic material, byco-melting, then cooling, the following metallic mixtures: a firstmetallic mixture (m1), which is: an alloy (a1), having a body-centredcubic crystal structure, comprising titanium, vanadium, and anothermetal M selected from chromium, manganese, and mixtures of those metals;or a mixture of the constituent metals of the alloy (a1), in theproportions of that alloy, those metals being present in the mixture inthe state of isolated metals and/or in the state of metal alloys; and asecond mixture (m2), which is an alloy (a2), comprising: from 38 to 42mole % of a first metal M¹ selected from zirconium, niobium, molybdenum,hafnium, tantalum, tungsten, and mixtures of those metals; and from 56to 60 mole % of a second metal M², selected from nickel, copper, andmixtures of those metals; or a mixture of the constituent metals of thealloy (a2), in the proportions of that alloy, those metals being presentin the mixture in the state of isolated metals and/or in the state ofmetal alloys; with a ratio by mass (m2)/(m1+m2) ranging from 0.1% to 20%by mass in the co-melting step; and (B) a hydrogenation of the compositemetallic material obtained, permitting the conversion of at least a partof the metals present into metal hydrides, and leading to afragmentation of the material in the form of a powder.
 19. Processaccording to claim 18, wherein the alloys (a1) and (a2) themselves areused as metallic mixtures (m1) and (m2), respectively.
 20. Processaccording to claim 18, wherein a single mixture (m1+m2) comprising thevarious constituent elements of the alloys (a1) and (a2) not alloyed inthe form of the alloys (a1) and (a2) is used as mixtures (m1) and (m2).21. Process according to claim 18, wherein the alloy (a1) corresponds tothe general formula (I) below:Ti_(a)V_(b)M_(c)M′_(d)   (I) wherein: M denotes Cr, Mn, or a mixture ofthose metals; M′ denotes a metal or a mixture of metals, other than Ti,V or Cr; a is a number ranging from 0.05 to 2.5; b is a number rangingfrom 0.05 to 2.9; c is a number ranging from 0.05 to 2.9; and d,optionally zero, is a number ranging from 0 to 0.5, the sum (a+b+c+d)being equal to
 3. 22. Process according to claim 18, wherein the alloy(a1) corresponds to the general formula (Ia) below:Ti_(x)V_(y)Cr_(3-(x+y))   (Ia) wherein: x is a number ranging from 0.4to 1; and y is a number ranging from 0.1 to 2.5, where the sum (x+y) isfrom 1.5 to 2.5.
 23. Process according to claim 22, wherein the alloy(a1) corresponds to one of the following formulae:TiV_(0.8)Cr_(1.2)Ti_(0.9)V_(0.7)Cr_(1.4) Ti_(0.833)V_(0.826)Cr_(1.334)Ti_(0.7)V_(0.9)Cr_(1.4);Ti_(0.66)VCr_(1.33)orTi_(0.5)V_(1.9)Cr_(0.6)
 24. Process according to claims 18, wherein thealloy (a2) corresponds to the formula (II) below:M¹ _(7-m)M² _(10-n)M³ _(p)   (II) wherein: M¹ denotes a first metal,selected from zirconium, niobium, molybdenum, hafnium, tantalum,tungsten, and mixtures of those metals; M² denotes a second metal,selected from nickel, copper, and mixtures of those metals; M³ denotes ametal or a mixture of metals, optionally present in the alloy, otherthan M¹ and M²; m is a positive or negative number, or zero, rangingfrom −0.1 to +0.1; n is a positive or negative number, or zero, rangingfrom −0.1 to +0.1; p is a positive number, or zero, ranging from 0 to0.2.
 25. Process according to claim 24, wherein the alloy (a2)corresponds to the formula (IIa) below:M¹ ₇M² ₁₀   (IIa) wherein M¹ and M² are as previously defined. 26.Process according to claim 25, wherein the alloy (a2) corresponds to thefollowing formula:Zr₇Ni₁₀.
 27. Process according to claims 1, wherein, in the co-meltingof step (A), the ratio by mass (m2)/(m1+m2) is from 0.5 to 15% by mass,preferably from 1 to 10% by mass.
 28. Process according to claim 18,wherein the co-melting of step (A) is carried out by induction heating,advantageously in an induction crucible.
 29. Process according to claim18, wherein step (B) is carried out by treating the composite materialobtained according to step (A) with hydrogen at a temperature of from 15to 100° C. and under a hydrogen pressure of from 0.8 to 1 MPa. 30.Composite metallic material obtainable at the end of step (A) of theprocess of claim 18, comprising: a majority phase comprising titanium,vanadium and chromium and/or manganese, dispersed in the form of grains,this phase having a body-centred cubic crystal structure; and at leastone intergranular phase comprising a first metal selected fromzirconium, niobium, molybdenum, hafnium, tantalum, tungsten, or amixture of those metals; and a second metal selected from nickel,copper, or a mixture of those metals.
 31. Composite metallic materialaccording to claim 30, wherein the grains have dimensions of from 10 to100 microns, preferably from 40 to 50 microns.
 32. Pulverulent materialcomprising metal hydrides that is obtainable at the end of the processof claim 18, by hydrogenation of the composite metallic material. 33.Method for the reversible storage of hydrogen in hybride form by use ofa pulverulent material according to claim
 32. 34. Pulverulent materialsubstantially free from hydrides, obtainable by dehydrogenation of apulverulent material according to claim 32.